1. Field of the Invention
The present invention relates to a tone conversion apparatus, an image display apparatus using the same, and a tone conversion method, which are used in a digital television (TV) broadcasting or the like.
2. Description of Related Art
In the digital TV broadcasting, 8-bit quantization has been performed so as to generate an 8-bit tone image signal. Further, to display an image based on the 8-bit image signal on a display device such as a liquid crystal display (LCD), a plasma display, or a planar projector, a dither circuit has been used to enable pseudo-representation of at least 8-bit tones. Also, as performance of the display device itself is improved, such a display device has been realized as to enable representation of at least 8-bit tones without using a dither circuit.
Japanese Patent Application Publication No. 2004-133260 discloses an image display method and an image display apparatus in which if converting an image signal into a multi-tone image signal, tone conversion device reads the multi-tone image signal from a lookup table based on a signal level of an input image signal, thereby enabling tone conversion of the image signal.
It is certainly possible to increase a tone easily by increasing the number of bits of an A/D converter for an analog signal to at least eight. Since, however, the digital TV broadcasting employs an 8-bit tone signal, if this signal is used in display, due to its fewer tones, a striped pattern appears in an image based on a Ramp signal or a smooth-gradation image with a suppressed change in luminance. In a case where tone conversion is performed using the lookup table as described above, if input image signals have a same signal level, the post-tone conversion image signals also have the same signal level. Therefore, in an image in which pixels having the same signal level run consecutively, it may be difficult to display an image having a smooth change in tone even by performing tone conversion.
Thus, it is desirable to provide a tone conversion apparatus, an image display apparatus using the same, and a tone conversion method, which can perform tone conversion such that an image can have a smooth change in tone.